P: Ability to include specific keyword during Export

See the Metadata Wrangler plugin.

This really needs to be driven by your particular requirements. Personally I would just apply the keywords as I go along, and then they will be already present when exported. They will merely assist searching etc in the meanwhile, but otherwise cause no harm. If their presence will cause you some problem for searching within the Catalog, though, there are some workarounds for that (which can be gone into separately).

It just seems arbitrary, laborious and error prone to me, to add keywords as part of an export, and then need to add those again when exporting again.

It is possible to do a keyword operation to the whole of an exported batch (say, as saved into an export preset for repeatability). This involves the utility Exiftool, as invoked (with suitable commandline arguments declaring the particular keyword operations) in the postprocessing section of the Export dialog. For example if your client had forbidden you to show their name anywhere on the photos within the Catalog, but required that to show on certain exported JPGs, this would be a way to achieve that: integrally, during creation of those exports. IMO that is an extreme example though.

Let's roll back and say there are keywords you sometimes want to include for export and other times want to exclude from export. This could be managed e.g. by making virtual copies into a new temporary collection, bulk removing the unwanted keywords as appropriate from those versions, exporting, then deleting those.virtual copies. Still not great.

So, another solution - assuming you are not otherwise relying on People tagging: you can check a box to classify the ones in question, as "People" keywords. (They don't actually need to be about people in any way, nor to relate at all to the people auto-detect function - which I personally have never touched: this is just a sneaky workaround). Now when you export, you can choose whether to have them or not as a class, via a special (exclude People data) override checkbox within the Export settings.